Automotive catalyst state control method

ABSTRACT

A control system and method for controlling an engine ( 10 ) of an automotive vehicle having a catalyst ( 34 ) is set forth herein. The control system maintains the efficiency of the catalyst by monitoring the catalyst state and driving the catalyst state to a target point. A first oxygen sensor ( 50 ) generates a first oxygen signal. A second oxygen sensor ( 52 ) downstream of the catalyst generates a second oxygen signal. A controller ( 12 ) is programmed to perform the steps of determining a catalyst state having a maximum value, a minimum value, and a target point therebetween; determining a commanded air-fuel ratio to drive the catalyst state to the target point; and operating the engine with the commanded air-fuel ratio.

TECHNICAL FIELD

[0001] The present invention relates generally to an exhaust gas control system for an internal combustion of an automotive vehicle, and more particularly, to a method and apparatus for controlling the catalyst efficiency by monitoring the state of the catalyst.

BACKGROUND

[0002] Minimizing tailpipe emission is an objective of closed loop fuel systems. Closed loop fuel systems include a catalytic converter that is used to treat the exhaust gas of an engine. The efficiency of a catalytic converter is affected by the ratio of air to fuel supplied to the engine. At the stoichiometric ratio, catalytic conversion efficiency is high for both oxidation and reduction conversions. The air-fuel stoichiometric ratio is defined as the ratio of air to fuel which in perfect combustion would yield complete consumption of the fuel. The air-fuel ratio Lambda of an air-fuel mixture is the ratio of the amount by weight of air divided by the amount by weight of fuel to the air-fuel stoichiometric ratio. Closed loop fuel control systems are known for use in keeping the air-fuel ratio in a narrow range about the stoichiometric ratio, known as a conversion window of an emission catalyst.

[0003] The difficulty with known systems is that the catalyst is very sensitive to errors in the input air fuel mixture that are less than the resolution of an upstream sensor output signal. Also, the oxygen storage capability of the catalyst can delay the response of a downstream sensor making the determination less accurate due to the time delay. One system estimates the oxygen mass stored in the catalyst by observing the amount of oxygen upstream of the catalyst and downstream of the catalyst to infer the catalyst capacity which may in turn be used to adjust the air-fuel ratio. Such systems suffer from the drawback mentioned above.

[0004] Other known systems use a predicted downstream oxygen sensor voltage to predict the amount of oxygen in the exhaust gas. One problem with this and other prior known systems is that the storage capacity of the catalyst will change with temperature and catalyst age. Therefore, the calculated efficiency in such systems may not correspond to the actual efficiency of the catalyst, particularly in older systems.

[0005] It would therefore be desirable to provide a method and apparatus for maximizing catalyst efficiency that takes into consideration the state of the catalyst.

SUMMARY OF THE INVENTION

[0006] The present invention provides a method and apparatus for controlling the operation of an engine of the automotive vehicle in response to a catalyst state rather than a calculation based on the amount of oxygen in the exhaust gas.

[0007] In one aspect of the invention, a method for controlling an engine comprises determining a catalyst state having maximum value, a minimum value and a target point therebetween. The method further comprises determining a commanded air fuel ratio to drive said catalyst state to the target and operating the engine with the commanded air fuel ratio.

[0008] In a further aspect of the invention, a system for controlling an engine of an automotive vehicle includes a catalyst as set forth herein. The control system maintains the efficiency of the catalyst by monitoring the catalyst state and driving the catalyst state to a target value. A first oxygen sensor generates a first oxygen signal. A second oxygen sensor downstream of the catalyst generates a second oxygen signal. A controller is programmed to perform the steps of determining a catalyst state having a maximum value, a minimum value, and a target point therebetween; determining a commanded air-fuel ratio to drive the catalyst state to the target; and operating the engine with the commanded air-fuel ratio.

[0009] One advantage of the invention is that factors such as temperature and age may be used in the catalyst state determination. This results in an improved efficiency calculation that does not correspond to a particular stored oxygen mass within the catalyst.

[0010] Other advantages and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a schematic view of a motor vehicle internal combustion engine together with apparatus for controlling the air-fuel ratio to the engine in accordance with the preferred embodiment of the invention.

[0012]FIG. 2 is a control block diagram of the catalyst state controller of the present invention.

[0013]FIG. 3 is a control block diagrammatic view of the catalyst state controller of FIG. 2.

[0014]FIG. 4 is a block diagrammatic view of the integrate state block of FIG. 3.

[0015]FIG. 5 is a control block diagrammatic view of the upstream reference block of FIG. 3.

[0016]FIG. 6 is a control block diagrammatic view of the Lambda control block of FIG. 3.

[0017]FIG. 7 is a block diagrammatic view of the catalyst capacity block of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0018] In the following example the same reference numerals and signal names will be used to identify the respective same components and the same electrical signals in the various views.

[0019] The present invention seeks to maximize catalyst efficiency based on the assumption that the catalyst is most efficient in an arbitrary state where the active catalyst sites are neither saturated with nor depleted of oxygen. Also, a high or low downstream exhaust gas oxygen sensor voltage indicates that the whole catalyst is effectively saturated rich or lean and is outside a catalyst state window. When the sensor indicate an intermediate value, both oxidation and reduction sites are available.

[0020] Referring now to FIG. 1, internal combustion engine 10 is controlled by electronic controller 12. Engine 10 has a plurality of cylinders 14, one of which is shown. Each cylinder has a cylinder wall 16 and a piston 18 positioned therein and connected to a crankshaft 20. A combustion chamber 22 is defined between piston 18 and cylinder wall 16. Combustion chamber 22 communicates between intake manifold 24 and exhaust manifold 26 via a respective intake valve 28 and an exhaust valve 30. Intake manifold 24 is also shown having fuel injector 32 coupled thereto for delivering liquid fuel in proportion to the pulse width of signal (FPW) from controller 12. The fuel quantity together with the amount of airmass in the intake manifold 24 defines the air-fuel ratio directed into combustion chamber 22. Those skilled in the art will also recognize that engine may be configured such that the fuel is injected directly into the cylinder of the engine in a direct injection type system.

[0021] A catalyst 34 is coupled to exhaust manifold 26 through exhaust system 36 and may comprise a three-way catalytic converter. Catalyst 34 is used to reduce tail pipe emissions by performing reduction and oxidation reactions with the combustion gasses leaving cylinder 22 through exhaust valve 30.

[0022] Controller 12 is shown as a conventional microcomputer 41 including a microprocessing unit (CPU) 38, input/output ports 40, read-only memory 42, random access memory 44, and a conventional data bus 46 therebetween.

[0023] Controller 12 is shown receiving various signals from sensors coupled to engine 10. The various sensors may include a mass airflow sensor 47 used to provide an airmass signal to controller 12. An engine speed sensor 48 is used to generate an engine speed signal corresponding to the rotational speed of the crankshaft. An exhaust gas oxygen sensor 50 positioned upstream of catalyst 34 provides a signal corresponding to the amount of oxygen in the exhaust gas prior to the catalyst. A second exhaust gas oxygen sensor 52 may be coupled to the exhaust system after catalyst 34. Sensors 50, 52 may comprise an EGO sensor, an UEGO sensor, or a HEGO sensor. Catalyst 34 may also have a temperature sensor 54 coupled thereto. Catalyst temperature sensor 54 provides an operating temperature signal for the catalyst to controller 12. Although a physical sensor 54 is illustrated, controller may also indirectly determine a temperature of the catalyst from other sensed inputs. The temperature of the catalyst may be estimated based upon the various engine operating conditions. In particular, catalyst temperature may be estimated using on a normal estimated temperature based on engine operating conditions that represent the catalyst temperature under normal conditions increased by a change in temperature based on the various operating conditions such as engine speed or load.

[0024] A throttle body 56 having a throttle plate 58 and a throttle position sensor 60 is illustrated. Throttle position sensor 60 provides controller 12 with an electrical signal corresponding to the desired driver demand.

[0025] Referring now to FIG. 2, a simplified block diagrammatic view of a portion of controller 12 is illustrated coupled to various blocks and illustrate various signals provided from the sensors to the controller 12. Block 70 is the catalyst state controller which is used to generate an air-fuel ratio lambse based upon the various inputs. The air-fuel ratio lambse is used to measure a catalyst state (CatState). The catalyst state is a normalized number between 1 and −1 indicative of a drift in the downstream oxygen level. That is, when the catalyst state is between 1 and −1, both reduction and oxidation sites in the catalyst are available. At 1 and −1, neither oxidation nor reduction sites are available. Lambse is a signal that averages to unity when the engine operates at stoichiometry with no air-fuel errors or offsets. Typically, lambse ranges between 0.75 and 1.25. A commanded state output (Out2) may also be provided from block 70 corresponding to the commanded state of the catalyst. Engine 12 receives the lambse signal and the airmass signal that controls the fuel injector as described above to provide an air-fuel ratio output to the engine (AirFuelOut). The combustion gasses are coupled to catalyst 54 and are output through the exhaust system at output one (Out1).

[0026] Inputs to block 70 include the output from exhaust gas sensor 50 upstream of catalyst 54 (UEGOLambda). The output of downstream exhaust gas sensor 52 is coupled to block 70 as DSHEGO volts. The airmass (AM) from throttle body 56 is also coupled to block 70. The catalyst temperature (CatTemp) is also coupled to block 70.

[0027] Referring now to FIG. 3, block 70 is illustrated in further detail. As described in FIG. 2, the outputs of block 70 are the catalyst state (CatState), lambse, and commanded state. The catalyst state (CatState) is determined in the IntegrateState block 72. The IntegrateState block 72 integrates the rate of the state change that results in an estimated catalyst state (CatState) between 1 and −1. The estimated catalyst state may actually extend beyond −1 and 1 due to estimation and measurement errors. Further details of the IntegrateState block 72 will be further described in FIG. 4 below. The IntegrateState block 72 has various inputs including airmass, lambda, lambda1KAM, current capacity, CatSaturated, and DSHEGOState inputs. The airmass is generated from the mass airflow sensor 47 in FIG. 1. Lambda is derived from the upstream exhaust gas sensor 50. Lambda1KAM is derived in upstream reference block 74. The current capacity of the catalyst is determined by catalyst capacity block 76. The catalyst saturated signal (CatSaturated) and the DSHEGOState signal are provided from state limits block 78. A lambda control block 80 receives the catalyst state output from IntegrateState block 72 and generates the CommandedState and lambse signals as will be further described below.

[0028] Block 76 receives the airmass signal and the catalyst temperature signal and determines a catalyst capacity (BaseCapacity). The units of capacity may, for example, be termed as delta lambda times the pounds of air.

[0029] Block 78 enables the IntegrateState in block 72 by monitoring the output of the downstream exhaust gas sensor 52 shown in FIG. 1. An Enable Integrate (Enab_Integrate) signal and a state reset value (StateResetVal) are generated. By monitoring the exhaust gas sensor 52, the determination whether or not the catalyst is saturated rich or lean may be determined. When the voltages exceed the predetermined limits, block 78 resets the states to 1 or −1 using the state reset value. Both sensor and stoichiometric chemistry errors are compensated for in block 74. The details of block 74 will be further described below.

[0030] Block 80 is coupled to the airmass signal and current capacity signal generated from block 76. Also, the target state is provided from a target state block 81. In the present example, the target state is set at zero. That is, between the catalyst states 1 and −1. Target state zero corresponds to the target state which will drive the catalyst to the most efficient state. Although zero is used, various numbers could be used and the number may also be adjustable based on load or other engine operating conditions.

[0031] Referring now to FIG. 4, IntegrateState block 72 of FIG. 3 is illustrated in further detail. The IntegrateState block 72 has a calculate state rate (CalcStateRate) block 82 that is used to determine the rate of change in the catalyst state. Block 82 receives lambda, lambda1KAM, airmass and current capacity signals as described above. The catalyst state dot (CatStateDot) is determined in block 82. The difference between lambda and lambda1KAM is the catalyst input air-fuel ratio error. Thus, the change in the catalyst state is determined as a function of the catalyst input air-fuel ratio error. That is, (CatStateDot)=[am*(Lambda−Lambda1Kam)]/Current Capacity. The output of block 82 is coupled with the catalyst saturated signal (CatSaturated) and the DSHEGOState signal. By integrating the catalyst state dot signal an estimate of the current catalyst state (CatState) is determined in block 84.

[0032] Referring now to FIG. 5, block 74 of FIG. 3 is illustrated in further detail. A target downstream voltage block 88 is coupled to a summing block 90. Summing block 90 is also coupled to the downstream heated exhaust gas oxygen voltage. The 0.6 value within the target downstream voltage block 88 represents the desired operating value of the downstream sensor. This value, of course, may change based upon a function of the engine operating condition such as engine speed and load.

[0033] The output of the summing block 90 is the downstream voltage error. Saturation block 92 receives the downstream voltage error. Saturation block 92 is used because the downstream voltage error may be larger in one direction than the other since the target voltage may not be exactly in the center of the sensor voltage range. Saturation block 92 limits the signal maximum and minimum values to provide a symmetric output.

[0034] A gain block 94 receives the downstream voltage error signal after passing through saturation block 92. The downstream voltage gain block 94 is coupled to integral controller 96 to calculate the lambda reference. Of course, those skilled in the art would recognize that other types of controllers may be used. As an alternative, the lambda reference may also be stored in a table as a function of engine load and speed. Other values may also be coupled to integral controller 96 to allow the proper integration of the signal and enable the integration. It should also be noted that the voltage gain of block 94 may be a function of catalyst capacity.

[0035] The lambda1 reference signal generated by integral controller 96 refers to the use of one upstream sensor. In various other applications, such as a “V” style engine, multiple sensors may be used.

[0036] As can be seen, a lambda reference is used to determine the measured upstream lambda that corresponds to the desired downstream heated exhaust gas oxygen voltage.

[0037] Referring now to FIG. 6, the CatState is used as an input to the fuel control where it is compared to a target catalyst state. FIG. 6 illustrates block 80 of FIG. 3 in further detail. The target state and the catalyst state are provided to a summing block 104. As mentioned above, the catalyst state is preferably between −1 and +1, while the target state is preferably a value therebetween such as zero. The difference in the signal is the catalyst state error (CatStateError) signal. This signal is provided to a proportional integrator controller (PI) having a proportional block 108 and a discrete-time integrator block 110. Those skilled in the art will recognize that other types of controllers could be used to control the target state and the estimated catalyst state. The PI controller 106 sums the proportional signal provided by block 106 in block 112. The sum of the proportional and integrated signal is the commanded state (CommandedState). The commanded state signal is added together with a constant from block 114 in block 116. The current capacity, the commanded state signal, and the airmass are combined together in product block 118 to obtain lambse. That is, by multiplying the commanded state times the current capacity and dividing it by the airmass, some limits are provided on the oxygen in and out rate. The storage reaction rate is limited and the high deviation from stoichiometric will have a tendency for breaking through the predefined limit. The commanded rate may therefore be clipped or limited by constant 114.

[0038] Referring now to FIG. 7, block 76 of FIG. 3 is described in further detail. In block 76, the capacity estimate of the catalyst is determined. It should be noted that if the catalyst estimate is in error the catalyst will not be operating at an optimum state contrary to the goals of the present invention. Various methods may be used to estimate the catalyst state. One manner would be to intrusively sweep the estimated state from one limit to the other preferably at a light engine loading condition where emissions are lowest. If the state saturates at opposite values to frequency this would be an indication that the estimated capacity is too large and thus the estimated capacity may be reduced. By compensating for the catalyst capacity, the age of the catalyst may also be taken into consideration. As the catalyst gets older, the capacity will be reduced. Thus, by determining the catalyst state having a maximum value, a minimum value, and a target point therebetween, a commanded air-fuel ratio may be determined to drive the catalyst to the target point. The engine is then operated with the commanded air-fuel ratio. 

What is claimed is:
 1. A method for controlling an engine coupled to a catalyst comprising: determining a catalyst state having maximum value, a minimum value and a target point therebetween; determining a commanded air fuel ratio to drive said catalyst state to the target point; and operating the engine with the commanded air fuel ratio.
 2. A method as recited in claim 1 wherein when the catalyst state is between said maximum value and said minimum value, determining a commanded air fuel ratio to drive said catalyst state to the target point.
 3. A method as recited in claim 2 when the catalyst state is not between said maximum value and said minimum value, determining a lambda error.
 4. A method as recited in claim 3 further comprising adjusting the commanded air-fuel ratio in response to said lambda error.
 5. A method as recited in claim 1 wherein generating said catalyst state is a function of measured lambda and a reference lambda, airmass and a current catalyst capacity.
 6. A method as recited in claim 1 further comprising the step of generating a current catalyst capacity as a function of airmass and catalyst temperature.
 7. A method as recited in claim 1 further comprising the step of generating a reference lambda corresponding to a stoichiometry value.
 8. A method as recited in claim 1 further comprising the step of when a downstream exhaust gas oxygen sensor value reaches a predetermined exhaust gas limit value, generating a reference lambda as a function of a lambda error.
 9. A method as recited in claim 1 wherein generating a reference lambda comprises determining a lambda error in response to airmass, catalyst state, and a previous catalyst state.
 10. A method as recited in claim 1 wherein said minimum value is about 1, said maximum value is about −1 and said set point is about zero.
 11. A method as recited in claim 1 wherein said target point is a function of load.
 12. A method as recited in claim 1 wherein said step of determining a commanded air fuel ratio comprises determining the commanded air-fuel ration as a function of airmass, current catalyst capacity and said target point.
 13. A method for controlling an engine coupled to a catalyst comprising: determining a rate of change of a catalyst state; estimating a current catalyst state by integrating the rate of change of the catalyst state; determining a commanded air fuel ratio to drive said catalyst state to a target point; and operating the engine with the commanded air fuel ratio.
 14. A method as recited in claim 13 wherein generating said catalyst state is a function of measured lambda and a reference lambda, airmass and a current catalyst capacity.
 15. A method as recited in claim 13 further comprising the step of generating a current catalyst capacity as a function of airmass and catalyst temperature.
 16. A method as recited in claim 13 further comprising the step of generating a reference lambda corresponding to a stoichiometry value.
 17. A method as recited in claim 13 further comprising the step of when a downstream exhaust gas oxygen sensor value reaches a predetermined exhaust gas limit value, generating a reference lambda as a function of a lambda error.
 18. A method as recited in claim 13 wherein generating a reference lambda comprises determining a lambda error in response to airmass, catalyst state, and a previous catalyst state.
 19. A method as recited in claim 13 wherein said target point is a function of load.
 20. A control system for an engine coupled to an emission catalyst having: a controller configured to determinr a catalyst state having maximum value, a minimum value and a target point therebetween; said controller further configured to determine a commanded air fuel ratio to drive said catalyst state to the target point; and said controller further configured to operate the engine with the commanded air fuel ratio.
 21. An article of manufacture comprising a computer storage medium having a computer program therein for controlling an engine coupled to a catalyst, said computer storage medium comprising: code for determining a rate of change of a catalyst state; code for estimating a current catalyst state by integrating the rate of change of the catalyst state; code for determining a commanded air fuel ratio to drive said catalyst state to a target point; and code for operating the engine with the commanded air fuel ratio. 